Print out the necessary manuals and strike off text you have already done during the work.
There will be four 2N3904 transistors in Tetrapolis. Measure their amplification factor and group them into two couples with similar amplification. Use one couple for receiver, one for transmitter.
Build the following modules:
Take thermal shields from the optical heads and mount them on receiver and transmitter, using 2 M3 nuts on each.
put four M4 (4.3mm) washers on receiver and four on transmitter. Press the optical head from the side and insert receiver and transmitter. Put on the slide covers and 4 M4 nuts on receiver and 4 M4 nuts on transmitter.
Put the heating cable that goes from the front cap through the single hole on the rear cap in both the optical heads. Connect it into the wire nut. Connect the receiver's and transmitter's lead into the terminal nut from the same side as you connected the heating cable.